Turning to FIG. 1, an example of a conventional illumination circuit 100 can be seen. In operation, the controller 104 provides a pulse width modulation (PWM) signal PWM1 to transistor Q1 (which, for example, is an NMOS transistor) so as to gate the transistor Q1. When activated, a regulated voltage source (which is represented as voltage VIN) provides a current that flows through the LED string 102 (which is generally comprised of LEDs D1 to DN coupled in series with one another) and resistor R1 (which can function, at least in part, as a current limiting resistor). By adjusting the PWM signal PWM1, the intensity of the emitted light from the LED string 102 can be varied, and this adjustment to the PWM signals PWM1 can be done in response to a measurement from photodiode PD. There are some problems with this arrangement, however; namely, use of the photodiode PD can substantially increase the cost and complexity of the illumination circuit 100. Thus, there is a need for an improved illumination circuit.
Some examples of conventional circuits are: U.S. Patent Pre-Grant Publ. No. 2006/0072319; and PCT Publ. No. WO2003083818.